The present invention relates to a radio communication device and, more specifically, relates to a signal receiver using a burst detector to obtain a timing reference.
A pulse communication receiver, such as a digital receiver or a radar receiver, must obtain a time reference to properly decode a received signal. A frequency burst can be detected in the received signal to provide the time reference. In a digital communication system, such as a GSM (Groupe Speciale Mobile) communication system, a burst of pure tone is periodically transmitted over a control channel. These bursts of tone are transmitted at a deterministic frequency offset from the channel center frequency (67.7 kHz above the center frequency in the GSM system) and are referred to as frequency correction bursts (FCBs). A timing reference for a channel band can be obtained by detecting the FCB at the deterministic frequency offset. Once an FCB has been detected, timing information can be extracted from the beginning or end of the FCB or other portions of the received signal, along with frequency information from the tone itself. This timing information can be used to synchronize the timing of a radiotelephone to a base station and to obtain timing for subsequent communications. Synchronization is required before detecting information to provide an output for the user of the receiver.
In a Time Division Multiple Access (TDMA) system such as in GSM, the FCB allows mobile units to align with the TDMA structure of the communication system as well as correct for any frequency offsets between the mobile unit and the base station by using the frequency information of the FCB tone. In order for these corrections to take place, the frequency location of the FCB must be determined with a high degree of accuracy.
When the transmitter and receiver have large frequency differences, the above timing correlation technique becomes unreliable. These large frequency differences can be caused by differences in the transmitter and receiver reference frequencies due to, for example, the crystal oscillator used as a master reference. Furthermore, a large frequency difference can be caused when the receiver moves relative to the transmitter at a large velocity. For example, an aircraft or a satellite is fast moving and typically would have Doppler frequency errors when communicating with a ground station or another aircraft or satellite. As the transmitter and receiver experience larger frequency differences, the received signal moves outside the range of correlation with the expected pattern. Thus, as the frequency difference increases, the received signal and expected pattern become increasingly decorrelated and hence it is more difficult to establish a timing reference.
For example, in the GSM system, frequency errors of the local oscillator (with respect to the base station) cause the FCB tone to appear at a frequency offset of 67.7+fd kHz, where fd is the frequency error between the mobile unit and the base station. Currently, with 900 MHz GSM radiotelephones, fd can take values up to xc2x125 kHz as the specified IF bandwidth is wide enough to detect frequencies up to 67.7xc2x125 kHz. However, for dual band operation including operation at the DCS 1800 MHz band, the same local oscillator stability now results in a frequency range of xc2x150 kHz. Present FCB detectors have difficulty extending to a frequency of xc2x150 kHz range. Further, a frequency range of xc2x150 kHz can extend significantly outside of the available IF passband, as shown in FIG. 1, making detection doubly difficult.
One method of improving the situation is to improve the crystal oscillator stability of the radiotelephone. However, providing tighter tolerance crystal oscillators is quite costly. Another method to address this problem is to widen the IF bandwidth of the radiotelephone. However, widening the IF filter causes poor detection rates in the presence of adjacent channel interference. Another method is to use adaptive filtering where the IF filter tracks frequency signals, as is known in the art. However, this technique is limited to existing detection boundaries as established for the GSM standard and requires additional circuitry and produces unwanted signals.
What is needed is a method and apparatus to extend the frequency detection range of FCBs without: widening IF bandwidth or adjusting the frequency of the IF filter, requiring tighter tolerance crystal oscillators, or requiring additional circuitry. It would also be a benefit to provide an extended FCB detection range without performance degradation.